1. Field of the Invention
The present invention relates generally to electrical controls for machinery and processes and more specifically to a method and apparatus for locking electrical control states automatically upon loss of electric power.
2. State of the Prior Art
Many machines and industrial equipment have functions that are monitored or controlled by electronic controls comprising microprocessors that monitor signals, physical conditions, or time and, in response to the presence or absence of such signals, physical conditions, or time, generate control signals to actuate or deactuate functions of the machines or industrial equipment according to some program, logic, or algorithm. The machines or industrial equipment respond to the presence or absence of such control signals to either initiate and perform some functions or to stop performance of some functions. For example, many conventional plastic injection molding machines have mechanisms that, upon sensing an electrical signal, start and continue through a cycle of closing a mold, filling the mold with liquid or molten plastic, cooling the mold to allow the plastic to harden into a hard plastic part, open the mold, and eject the hard plastic part out of the mold. A monitoring system senses whether the hard plastic part has been successfully and completely ejected from the mold before the injection molding machine is allowed to close the mold again and to proceed with the cycle to make another plastic part, because mistakenly closing the mold on a partially ejected hard plastic part that has not been cleared from the mold can cause severe and costly damage to the mold.
Loss of electrical power in such monitoring systems can leave control signals for such machines or equipment in undesirable and perhaps even unsafe states. For example, in the plastic injection molding machines described above, a power loss in the monitoring system could leave an output signal in a state that allows the injection molding machine to close the mold, even if a hard plastic part has not been successfully ejected and cleared from the mold, which, as mentioned above, can cause severe and costly damage to the mold.
There are many kinds of equipment and control schemes available to provide myriad degrees of protection against undesired consequences in electric power loss situations for various applications. For example, some equipment incorporates a power down cycle in which the equipment senses a power-off request signal from a switch actuated by an operator and, in response, takes the time to run through a series of steps to put itself in a safe state before actually turning off the power via an electronic device. However, if electric power is disconnected or lost to such equipment unexpectedly, such as a power line failure, its outputs may be left in undefined states that might be dangerous to related equipment or to operators.
In another approach, plug-in output modules have been used with equipment, which modules can be selected to have a desired combination of normally-opened and/or normally-closed states in the absence of power to provide safe conditions in the equipment. This approach, however, lacks flexibility. If the equipment is used in a different application where such combination of normally-opened and/or normally-closed states yields unsafe or undesired conditions, the equipment would have to be re-wired with a new set of modules.
Still another approach used in some monitoring or control devices has been to use a power-fail output from the monitoring or control devices to "warn" companion equipment that are being monitored or controlled that the monitoring or control devices are not functional. Then, it is left to the equipment that was being monitored or controlled to fend for itself, whether it be to continue functioning automatically or to shut-down in response to such "warning."
Yet other equipment have been designed and built to be inherently fail-safe regardless of the outputs from associated monitoring or control devices. However, such equipment that is specially designed to deal with unexpected or improper outputs from associated monitoring or control devices are probably more expensive than would be needed if another solution was available for power loss situations.